1. Field of the Invention
The present invention relates generally to a battery charge control circuit, and more particularly, to a battery charge control circuit that adjusts the charging of a battery depending on a difference in voltage between a supply voltage and an output charge voltage.
2. Description of the Related Art
Conventionally a battery charge control circuit like that shown in FIG. 4 is known. As shown in FIG. 4, the conventional battery charge control circuit has a line terminal 10 to which a supply voltage Vcc is supplied, with the battery to be charged connected to an output terminal 12. The line terminal 10 is connected to the emitter of a PNP-type output transistor Q1. The collector of transistor Q1 is connected to the output terminal 12 via a current-sensing resistor R1.
Within the semiconductor integrated circuit, between a ground line 16 and a power line 14 connected to the line terminal 10, a constant-current power supply 18 and a Zener diode 20 are connected in series. A reference voltage Vref is generated at the Zener diode 20. Additionally, one end of a resistor R3 is connected to the constant-current power supply 18, the other end of the resistor R3 being connected to the collector of an NPN-type transistor Q3. Transistor Q3 is connected across a collector-base gap and at the same time that base is jointly connected to the base of an NPN-type transistor Q4, the respective emitters of transistors Q3 and Q4 being grounded to form a current mirror circuit.
The collector of transistor Q4 is connected to the collector and base of PNP-type transistor Q5. Transistor Q5 is jointly connected to the respective bases of transistors Q6, Q7, Q8 and Q9, the emitters of transistors Q5, Q6, Q7, Q8 and Q9 being connected to the power line 14 to form a current mirror circuit. Transistor Q9 supplies an actuating current to an operational amplifier 22, transistor Q8 supplies an actuating current to a comparator 24, transistor Q7 supplies an actuating current to a constant-voltage power supply 28 and transistor Q6 supplies an actuating current to a differential amplifier 26.
The differential amplifier 26 comprises the operational amplifier 22 together with resistors R4, R5, R6 and R7. The differential amplifier 26 differentially amplifies an output voltage Vbat of output terminal 12 supplied to an inverted input terminal via resistor R4 and a supply voltage Vcc supplied to a noninverted input terminal via resistor R6, and supplies that differential voltage to a noninverted input terminal of the comparator 24. The reference voltage Vref generated at the Zener diode 20 is supplied to the inverted input terminal of the comparator 24, the comparator 24 outputting a LOW signal when the differential voltage is less than the reference voltage Vref and a HIGH signal when the differential voltage is greater than the reference voltage Vref.
The signal output from the comparator is supplied to the base of a control transistor Q2, the control transistor Q2 being forcibly cut off when the differential voltage is less than the reference voltage Vref. This forcible cutting off of the control transistor Q2 when the differential voltage is less than the reference voltage Vref is provided in order to prevent current from flowing in reverse from the battery to this circuit when the line terminal 10 voltage is less than the output terminal 12 voltage during insertion and removal of the power supply Vcc adapter from the charge circuit.
The constant-voltage power supply 28 is supplied with an actuating current and generates a constant voltage Vreg that is then supplied to the noninverted input terminal of the differential amplifier 26. A voltage Vbat from the output terminal 12 is supplied to the inverted input terminal of the differential amplifier 26. The differential amplifier 26 then supplies the difference in voltage between Vbat and Vreg to the base of control transistor Q2. As a result, when the comparator output is HIGH, control transistor Q2 transmits a collector current corresponding to the difference in voltage between Vbat and Vreg and output transistor Q1 adjusts the output current based on that collector current, so that the voltage Vbat of output terminal 12 is adjusted to a value corresponding to a constant voltage Vreg.
The conventional battery charge control circuit shown in FIG. 4 requires operational amplifier 22 and resistors R4 through R7 as well as comparator 24 in order to determine whether or not the difference in voltage between the supply voltage Vcc and the output voltage Vbat exceeds a reference voltage Vref, thus requiring a large number of elements and thereby increasing the size of the circuit. Additionally, further cumbersome measures must be taken to ensure that the operational amplifier 22 and comparator 24 are started before the differential amplifier 26 and constant-voltage power supply 28 in order to prevent improper operation at start-up.